This invention relates to recovering sulphur from a gas stream comprising ammonia and hydrogen sulphide.
Waste gas streams comprising hydrogen sulphide and ammonia are frequently encountered in refineries. Because hydrogen sulphide and ammonia are poisonous gases such waste gas streams need to be appropriately treated before being discharged to the atmosphere. Although such gas streams can be employed as a feed stream to the Claus process, care has to be taken to ensure that all the ammonia is destroyed upstream of the catalytic stages of the process because residual ammonia tends to react with sulphur dioxide to form ammonium salts which will block or poison the catalyst. These problems tend to increase in severity with increasing ammonia concentration in the waste gas stream.
As a result, particularly if the ammonia concentration in the waste gas stream is in excess of 30% by volume, the practice in the art is to employ two separate combustion zones at the front end of the Claus process. All the waste gas stream comprising ammonia and hydrogen sulphide is fed to an upstream combustion zone. The waste gas stream is typically mixed with one part of another waste gas stream comprising hydrogen sulphide but essentially no ammonia. The rest of the other waste gas stream is supplied to a downstream combustion zone. Accordingly any ammonia which is not destroyed in the upstream combustion zone will tend to be incinerated in the downstream combustion zone. Such processes are, for example, disclosed in WO-A-88/02350 and EP-A-0 325 286.
EP-B-0 034 848 discloses destroying the ammonia content of a waste gas stream by supplying the gas stream to the outer of two concentric tubes forming part of a burner. A hydrogen sulphide stream free of ammonia is supplied to the inner concentric tubes. The two tubes debouch into a mixing chamber, of which the downstream end terminates in a combustion chamber. Although only a single combustion zone is nominally employed, difficulties arise in the fabrication and operation of the mixing chamber such the high temperatures created do not damage it. The reason for employing the mixing chamber is to ensure that the gases to be burned are thoroughly mixed with combustionxe2x80x94supporting air upstream of the combustion chamber. Intimate mixing is deemed to be necessary to ensure that all the ammonia is destroyed by combustion.
We have discovered that the thermal dissociation of ammonia to nitrogen and hydrogen can play an important part in its destruction. Accordingly, provided an adequately high temperature region or regions within the flame zone can be created for the thermal cracking of ammonia it is not necessary either to employ two separate combustion or flame zones or, in the case of a single flame zone, to employ a discrete mixing chamber upstream thereof.
According to the present invention there is provided a method of recovering sulphur from a first gas stream comprising hydrogen sulphide and at least 50% by volume of ammonia and from a second gas stream comprising hydrogen sulphide but essentially no ammonia, including feeding the first gas stream, the second gas stream, and combustion supporting gas comprising at least one stream of essentially pure oxygen or oxygen-enriched air to a single combustion zone or a plurality of combustion zones in parallel with each other within a reactor without premixing of combustible gas with oxygen or air, and creating in the or each combustion zone at least one region in which thermal cracking of ammonia takes place, and taking from the reactor an effluent gas stream including sulphur vapour, sulphur dioxide, and hydrogen sulphide, but essentially no residual ammonia.
If desired a single burner or a plurality of burners may fire into the or each combustion zone.
The method according to the invention is particularly suitable for use if the first gas stream contains at least 60% by volume of ammonia.
Preferably, there is fed to the or each combustion zone in addition to the stream or streams of essentially pure oxygen or oxygen-enriched air a stream or streams of air. Such an arrangement facilitates the creation of a relatively hot thermal cracking zone or zones within the combustion zone without exceeding a maximum temperature for the effluent gas stream above which thermal damage is liable to be caused to the reactor even if the normal precaution of providing the reactor with an internal refractory lining is taken.
Preferably, the or each combustion zone has at least three stages. In one arrangement of a burner for use in the method according to the invention to create a combustion zone having at least three stages, a first flow of the first gas stream is preferably supplied to the flame from a first region of the mouth of the burner; at least one second flow of a combustion supporting gas is caused to issue from the mouth of the burner and mix in the flame with the first gas stream; at least one third flow of the second gas stream is supplied to the flame from a second region of the mouth of the burner surrounding and spaced from the first region; at least one fourth flow of a combustion supporting gas is caused to issue from the mouth of the burner the and mix in the flame with the second gas stream, and at least one fifth flow of a combustion supporting gas of different composition from the second and fourth flows is caused to mix in the flame with the second gas stream. Burning the first and second gas streams in three stages, typically an innermost stage, an outermost stage, and an intermediate stage, makes it possible to achieve a relatively low temperature in the outermost stage in comparison with a temperature in excess of 2000xc2x0 C. in the innermost stage. Such a high temperature in the innermost stage facilitates destruction of the ammonia in the first gas stream.
Preferably the flame extends generally longitudinally within the furnace. The furnace is typically disposed with its longitudinal axis horizontal, and therefore the burner is typically also disposed with its longitudinal axis extending horizontally. Such arrangements can help to keep down the risk of damage to any refractory lining employed in the furnace.
The second and fourth flows of combustion supporting gas preferably both have a mole fraction of at least 0.22 and may be oxygen-enriched air containing at least 50% by volume of oxygen or pure oxygen. The third oxidising gas is preferably atmospheric air neither enriched in nor depleted of oxygen, although enrichment up to 25 or 30% by volume of oxygen, or higher depending on the composition of the first and second gas streams, is generally acceptable.
Mixing of the first gas stream with the first combustion supporting gas is preferably facilitated by directing at least some of the first combustion supporting gas along a path or paths which meet a path or paths followed by the first gas streams. Accordingly, the second outlet or at least some of the second group of outlets preferably each have an axis which extends at an angle to the axis of the first outlet or the axes of at least some of the second group of outlets. The angle is preferably in the range of 10 to 30xc2x0. Preferably, the flow of the first gas stream is axial and the flow of the first combustion supporting gas is at an angle to the axis of the burner. The combustible gas stream and the first gas stream may be supplied at the same velocity as one another or at different velocities.
Alternatively, mixing of the first gas stream with the first combustion supporting gas can be facilitated by directing at least some of the first combustion supporting gas at a first linear velocity along a path or paths generally contiguous and generally parallel to a paths or paths followed by the first combustible gas at a second linear velocity, and one of the first and second linear velocities is from 25 to 150% (and preferably from 25 to 100%) greater than the other thereof. Mixing is facilitated because the differential velocity between the first combustion supporting gas and the first gas stream creates forces of shear therebetween. Preferably, it is the first linear velocity which is selected to be a greater of the two velocities. This arrangement facilitates design of the furnace to ensure that all the ammonia is destroyed in it. A further alternative or additional means for facilitating mixing of the first gas stream of the first oxidising gas is to impart a swirling motion to one or both of the streams. Devices which are able to impart swirl to such gas are well known.
The natural curvarture of the flame tends to facilitate mixing of the forth flow of second oxidising gas with the third flow of the second gas stream. Nevertheless, it is preferred to arrange the supply of the said third and fourth flows so as further to facilitate mixing. Similar means to those described above with reference to the first and second flows can therefore be used.
A particular advantage of the method according to the invention is that any oxides of nitrogen formed in the innermost or other oxygen rich stage of the flame will be reduced back to nitrogen with the result that the effluent gas stream is essentially free of oxides of nitrogen.